Modal verbs

Must / can't / couldn't have - to express probability in the past

Structure: modal + have + past participle
must have been, can't have gone, etc.

We use must have to express that we feel sure that something was true.

They must have left early.
He must have already gone.

We use can't have / couldn't have to say that we believe something was impossible.

He can't have escaped through this window. It is too small.
She can't have said that.
She couldn't have said that.

May / might / could have - to express probability in the past

Structure: modal + have + past participle 
may have been, could have gone, might have lost, etc.

We use may / could / might have to say that it was possible that something happened in the past (but we are not 100% sure).

He may have missed the bus.
The road might have been blocked.

The negatives are may not have and might not have.

He may not have left yet.
The assistant might not have received his message.

Must / can't - to express probability in the present

Structure: modal + infinitive without to
must be, must have, can't go, etc.

We use must to express that we feel sure that something is true.

They are really good, they must win. 
They must be very rich. Look at the house.

We use can't to say we are sure that something is impossible.

She can't be ill. I've just seen her in the shop and she looked fine. 
It can't be true. I don't believe it.

May / might / could - to express probability in the present

Structure: modal + infinitive without to
may be, might do, could go, etc.

We use may or could or might to say that it is possible that something will happen or is happening.

They may be arriving tomorrow.
He might be away on holiday.
He could be away on holiday. 
He might be offered the job.

The negative of may is may not.
The negative of might is might not.

They both mean that it is possible that something will not happen or is not happening.

We DO NOT use could not to express probability.

He might not be offered the job. 
I may not pass the exam.
I might not go to the match tomorrow.
I  go to the match tomorrow.